The invention relates to a preamplifier having an input terminal for receiving a periodically varying input signal to be amplified, which input terminal is connected to at least one control electrode of a voltage-controlled transistor and to a terminal of a resistor, the transistor and the resistor forming part of a first stage of the preamplifier, an output electrode of this transistor being coupled to an input terminal of a second stage of the preamplifier via a direct current connection, an output terminal of the second stage being coupled to a further terminal of the resistor via a direct current connection, which preamplifier has an output terminal for supplying the amplified input signal, the preamplifier comprising a fedback signal clamping circuit which is fedback from the output terminal of the preamplifier for periodically clamping a signal level in the amplified signal at this output terminal at a reference voltage.
Such a preamplifier employed in a television camera is described in "Journal of the SMPTE", July 1973, pages 547 to 551, inclusive, more specifically on pages 548 (FIG. 4) and 549 (FIG. 6). The voltage-controlled transistor, which has an insulated gate electrode, is arranged near a television pick-up arrangement to which the gate electrode, being the control electrode, is connected. The input signal of the preamplifier is a television picture signal. The drain or output electrode of the transistor and the adjacent resistor are connected via a multi-wire cable which acts as a direct current connection, to the terminals of the second stage of the preamplifier, which stage is located at some distance. A third stage of the preamplifier formed by differential amplifiers is arranged subsequent to the second stage which comprises discrete transistors, an output terminal of the third stage forming the amplified picture signal-carrying output terminal of the preamplifier. The signal clamping circuit, which is fed from the output terminal of the preamplifier, is fedback to an input terminal of the second stage. When the preamplifier is used for television, as described, the signal clamping circuit causes, in the amplified picture signal, the black level present in line blanking periods to be clamped onto the reference voltage.